Typical prior art variable reluctance rotation sensors are positioned adjacent a rotatable ferromagnetic target wheel having teeth separated by slots along its periphery. The target wheel is coupled to and rotated by a rotating shaft, the rotational speed and/or position of which is desired. These variable reluctance sensors generally comprise a permanent magnet attached to one end of an elongated core made of soft magnetic material, about which is wound an electrical coil, which projects outwardly from the magnet away from the target wheel. The permanent magnet and target wheel periphery define an air gap which varies as the target wheel rotates. The air gap is at a minimum when the sensor is directly adjacent a "tooth," and at a maximum when directly adjacent a "slot."
The permanent magnet produces a magnetic field which drives flux across the air gap, through a portion of the target wheel, back across the air gap, through the electrical coil, into the elongated core and back to the permanent magnet. As the target wheel rotates, the varying air gap varies the reluctance of the flux path, which in turn varies the flux magnitude passing through the elongated core. This varying flux produces an AC voltage in the electrical coil having an amplitude proportional to the rate of change of flux and a frequency proportional to the rotational speed of the target wheel.
The objective is to obtain a voltage output signal having a peak- to-peak amplitude sufficiently large for easy sensing and signal processing, while at the same time minimizing the size of the permanent magnet and the associated magnetic path in order to permit the manufacture of small, compact sensors. The peak-to-peak amplitude of the voltage is dependent upon the strength of the permanent magnet (i.e., amount of magnetic flux it generates), the size of the air gap, the number of turns in the electrical coil, and the rotational speed of the target wheel.
Modern anti-lock braking and traction control systems require incorporation of rotational sensors in vehicle wheel bearings for detection of wheel rotation. Due to packaging constraints, attempting to increase the output voltage by increasing the number of coil turns necessitates using smaller wire, which is more prone to breakage during manufacture of the sensor and operation of the vehicle. Packaging constraints also limit the size of the permanent magnet, which can necessitate the use of expensive, high strength rare earth permanent magnets to drive enough flux across the air gap and through the elongated core to achieve the required output voltage.
Air gap variations between the sensor and target wheel also have a significant affect upon the output voltage. The closer the sensor is placed to the periphery of the target wheel, the greater the peak-to-peak amplitude of the output voltage signal. Accordingly, the larger the minimum air gap, the lower the peak-to-peak amplitude of the output voltage signal. Unfortunately, manufacturing and assembly variations may result in large minimum air gaps, lowering the voltage output.
Additionally, the peak-to-peak amplitude of the voltage output decreases with the rotational speed of the target wheel since the rate of change of the flux due to air gap differences decreases as the teeth move more slowly past the sensor.
In light of the aforementioned, small, inexpensive, durable variable reluctance rotation sensors generating high output voltages in the presence of low rotational speeds and/or large air gaps are not readily available.